Perhaps, but all of these things must be measured in the benefit to business. Does the performance between a 32-bit or 64-bit versions translate to a difference in sales? Are there user complaints sufficient that there is lost satisfaction to competitors large enough to offset the development and support burden from the investment in a 64 bit client? My guess is that the answer is “no” or Valve would have made this change already. These are the way businesses make these decisions.